Laser rangefinder sensor

ABSTRACT

The specification discloses a pulsed time-of-flight laser range finding system used to obtain vehicle classification information. The sensor determines a distance range to portions of a vehicle traveling within a sensing zone of the sensor. A scanning mechanism made of a four facet cube, having reflective surfaces, is used to collimate and direct the laser toward traveling vehicles. A processing system processes the respective distance range data and angle range data for determining the three-dimensional shape of the vehicle.

CROSS-REFERENCE

The present specification relies on U.S. Provisional Patent ApplicationNo. 61/534,148, filed on Sep. 13, 2011 and entitled “Improved LaserRangefinder Sensor”. The aforementioned specification is incorporated byreference in its entirety herein.

FIELD

The present specification relates generally to object sensors, and inparticular to improved laser rangefinder sensors useful in accuratelyand precisely sensing, detecting and/or classifying vehicles while alsoenabling the triggering of traffic, import, export, or other regulatoryenforcement cameras.

BACKGROUND

A conventional optoelectronic sensor uses a time-of-flight laserrangefinder system to measure the normal distance to a road surface froma fixed point above the road surface and then measure the distance to avehicle which either passes or stops under the sensor. Because of thehigh repetition rate of the pulsed beam, traditional systems are able todevelop a longitudinal profile of the vehicle using multiple consecutiverange measurements as the vehicle moves under the sensor. Someconventional systems may also be able to determine vehicle speed and usethis information to develop a profile of the vehicle.

Conventionally, the sensor receives a portion of the energy reflectedfrom either the area or an object located within the area, such as avehicle. The returned pulse energy is then provided as an input to areceiver for determining a time of flight change for pulses emitted andreceived, which may be caused by the presence of an object within thearea. The sensor is also provided with various features useful inproviding outputs which indicate the speed, census, size or shape of oneor more objects in the area. For example, a typical sensor is providedwith a component for receiving an input from the time of flightdetermining means and for providing an output indicating whether theobject meets one of a plurality of classification criteria (e.g., is theobject an automobile, truck or motorcycle).

Such sensors are being used as noninvasive solutions to track andanalyze traffic across a wide range of applications, including tollcollection, traffic flow analysis, bridge/tunnel clearance verification,as well as traffic control and surveillance. These applications havehighly dynamic operating environments that demand very precise sensortracking and detection capabilities. Conventional systems are stillunable to accurately measure and track high speed traffic flow through alocation with sufficiently high scan rates to enable vehicleidentification and classification, particularly during inclementweather.

Accordingly, there is need for a sensor system with improved rangeaccuracy and resolution at high scan rates. There is also a need for asensor system that reduces false measurements arising due to adverseweather conditions.

SUMMARY

The present specification discloses a pulsed time-of flight rangingsensor comprising laser means for providing vehicle classificationinformation. More specifically, the present specification discloses apulsed time-of-flight ranging sensor comprising laser means fordetermining a distance range from the sensor to portions of a vehiclewhereby the vehicle travels within a sensing zone of the sensor. Thepresent specification also discloses respective range data outputscorresponding with a sensor angle for each distance range data output.In addition, scanning means for scanning at least one beam across thevehicle is provided, which in one embodiment is a four facet cube,having reflective surfaces, that is used as a scanning mirror. Further,processing means is also provided for processing the respective distancerange data and angle range data for determining the three-dimensionalshape of the vehicle.

In one embodiment, the present specification is a system for determiningthe three-dimensional shape of a vehicle, the system comprising: adistance sensor comprising a laser transmitter and a photodetector, forgenerating a plurality of laser beams and for detecting a plurality ofreflected beams, each of said reflected beams corresponding to one ofthe plurality of generated laser beams; a scanning mechanism, positionedrelative to the distance sensor, for collimating each of said generatedlaser beams across the vehicle, wherein said scanning mechanismcomprises a four facet cube, each having a reflective surface, whereinsaid four facet cube is positioned relative to the distance sensor suchthat it is adapted to reflect the generated laser beams, and whereinsaid scanning mechanism further comprises a scanner control circuit indata communication with said distance sensor to trigger the generationof the laser beams to create predefined scan angles; and a processingsystem to determine distance ranges from the sensor to portions of thevehicle using time-of-flight measurements derived from timings of saidgenerated laser beams and reflected beams, when the vehicle travelswithin a sensing zone of the sensor and to determine a three-dimensionalshape of the vehicle based on distance ranges.

Further, the system of the present specification comprises atime-to-digital converter (TDC) for time-of-flight measurements, whereinthe TDC is adapted to receive up to four return pulses from a singlelaser pulse. In one embodiment, the system comprises at least two TDCs.

In one embodiment, the four facet cube described in the presentspecification rotates continuously in one direction at a constant speedand enables four scans for each revolution.

In one embodiment, the system generates a plurality of laser footprintsand wherein said laser footprints appear as stripes that touch end toend and provide a continuous line of detection.

In one embodiment, the present specification discloses a method fordetermining a three-dimensional shape of a vehicle passing through asensing zone of a ranging sensor comprising a laser transmitter and aphotodetector, the method comprising: scanning a plurality of laserbeams across the vehicle using a scanning mechanism comprising a fourfacet cube, said four facet cube having reflective surfaces that areused to direct the laser beams across its field of view in a straightline, said scanning mechanism further comprising a scanner controlcircuit that triggers the laser at predefined scan angles; determining adistance range from the sensor to portions of the vehicle usingtime-of-flight measurements; and processing the distance range data foreach scan angle to determine the three-dimensional shape of the vehicle.

In one embodiment, the four facet cube rotates continuously in onedirection at a constant speed during scan. In another embodiment, thescanning cube is adapted to produce four scans for each revolution.

In one embodiment, the method of the present specification further usesa time-to-digital converter (TDC) for time-of-flight measurements,wherein the TDC is adapted to receive up to four return pulses from asingle laser pulse. In another embodiment, the system comprises at leasttwo TDCs.

In one embodiment, the system generates a plurality of laser footprintsduring a scan and wherein each of said laser footprints appear asstripes that touch end to end and provide a continuous line ofdetection.

In one embodiment, the distance range resolution of the system is ±1 cm.In another embodiment, the limits of distance range measurements arecustomizable.

In one embodiment, the scanner control circuit triggers a laser pulseonce for every degree of scan angle.

The aforementioned and other embodiments of the present specificationshall be described in greater depth in the drawings and detaileddescription provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will befurther appreciated, as they become better understood by reference tothe detailed description when considered in connection with theaccompanying drawings:

FIG. 1 shows a front-elevation view of one embodiment of anoptoelectronic sensor system mounted above a roadway;

FIG. 2 is a side-elevation view of the sensor, described in FIG. 1A,mounted at a height ‘H’ and at a forward look down angle ‘A’;

FIG. 3 shows the laser footprint, in one embodiment, wherein the sensoris mounted at a height of approximately 7 meters;

FIG. 4 is a table providing lane coverage data according to varyingmount heights for the sensor when the sensor is angled at 0 degrees withrespect to the direction of traffic;

FIG. 5A is a table showing a plurality of performance parameters for thesensing system in accordance with one embodiment;

FIG. 5B is a table describing a plurality of laser output parameters inaccordance with one embodiment of the present invention;

FIG. 5C is a table showing a plurality of environmental factors andassociated performance parameters in which the sensor is capable ofperforming, in accordance with one embodiment of the present invention;and,

FIG. 6 illustrates a rotating cube scanner according to one embodimentof the present invention.

DETAILED DESCRIPTION

The present specification discloses a pulsed time-of flight rangingsensor system comprising laser means for providing vehicleclassification information. More specifically, the present specificationdiscloses a pulsed time-of-flight ranging sensor system comprising alaser means for determining a distance range from the sensor to portionsof a vehicle whereby the vehicle travels within a sensing zone of thesensor and a respective range of data outputs corresponding with asensor angle for each distance range data output.

In addition, the sensing system comprises a scanning means for scanningat least one beam across the vehicle, which, in one embodiment, is afour facet cube, having reflective surfaces, that is used as a scanningmirror. Further, a processing means is also provided for processing therespective distance range data and angle range data for determining thethree-dimensional shape of the vehicle.

The present specification discloses multiple embodiments. The followingdisclosure is provided in order to enable a person having ordinary skillin the art to practice the inventions. Language used in thisspecification should not be interpreted as a general disavowal of anyone specific embodiment or used to limit the claims beyond the meaningof the terms used therein. The general principles defined herein may beapplied to other embodiments and applications without departing from thespirit and scope of the invention. Also, the terminology and phraseologyused is for the purpose of describing exemplary embodiments and shouldnot be considered limiting. Thus, the presently disclosed inventions areto be accorded the widest scope encompassing numerous alternatives,modifications and equivalents consistent with the principles andfeatures disclosed. For purpose of clarity, details relating totechnical material that is known in the technical fields related to theinvention have not been described in detail so as not to unnecessarilyobscure the present invention.

FIG. 1 shows a front-elevation view of an optoelectronic sensor 105 of asensing system that is mounted above a roadway 110, having a pluralityof travel lanes 115, for sensing, detecting, and/or classifying vehicles120 passing below the sensor 105 and also for triggering video and/oraudio capturing equipment. In accordance with one embodiment, the sensor105 is used in multilane electronic toll collection operations fordetecting vehicles travelling at expressway speeds. The sensor 105 istypically mounted overhead of travel lanes on either a gantry, pole armor toll plaza roof structure 125. More specifically, the sensor ispreferably mounted in or around the center point of a horizontal portionof the gantry structure 125 which extends over and above the roadway 110and is kept aloft by being fixedly attached to right and left verticalportions of the gantry structure 125.

FIG. 2 is a side-elevation view of the sensor 205 mounted at a height‘H’ of approximately 7 meters (23 ft) and at a forward look down angle‘A’ 215 of 10 degrees. The sensor is preferably mounted offset fromnormal such that a beam 225 emitted from the sensor 205 travels downwardtoward the roadway and intersects the roadway at an angle that, relativeto the gantry 230, is less than 90 degrees. Such an angle can be formedby having the sensor 205 mounted, relative to the gantry 230, using alook down angle 215 in a range of less than 25 degrees and preferablyapproximately 10 degrees.

Referring to FIGS. 1 and 2, during operation, the sensor 105, scans theroadway, taking distance/range measurements across the width of the roadbeneath the sensor. When no vehicles are present, the range measurementsare equal to the distance range to the road 110. When a vehicle 120 ispresent beneath the sensor, the distance to the top surface of thevehicle is measured and provides a transverse height profile of thevehicle on each scan. Thus, when a vehicle passes through the scanninglaser beam, as shown in FIG. 2, the distances or ranges to variouspoints on the surface of the vehicle are measured by emitting aplurality of laser beams toward the vehicle, detecting a correspondingreflecting beam for each of the plurality of laser beams, recording thetime of flight for each emitted and corresponding reflecting beam andusing the time of flight data to generate distance information. In oneembodiment, sensor 105 scans a narrow laser beam 135 across the width ofthe 90 degree field of view at a rate of 120 scans per second (sps). Thenarrow laser beam width enables detection and separation of closelyfollowing vehicles traveling at high speed.

These ranges or measured distances are then used in generating a vehicleprofile. The profile is formed by using geometric transformations, wellknown in the art, for the distance measurements obtained. In oneembodiment, the laser scan is carried out at various scan angles toobtain a wider range of distance measurements, and to generate a moreaccurate vehicle profile.

In one embodiment, these measurements are streamed (using wired and/orwireless network) real-time to a computer that is programmed to uniquelydetect, classify and determine the position of each vehicle in theroadway. In accordance with an aspect of the present invention, thescanning laser rangefinder measures a single plane profile that enablesimproved accuracy of vehicle detection and triggering. In oneembodiment, pulsed time-of-flight range measurements provide accurate±2.5 cm (±1.0 in.) vehicle profiles. By streaming consecutive scans tothe computer, a full three-dimensional vehicle profile can be developedin real-time.

It should be noted herein that the sensor mounting height may vary witheach installation site. Various horizontal beam width and mountingheight correlations are provided below and shown with respect to FIG. 4,which provides a table providing lane coverage, in terms of horizontalbeam width 405, with respect to varying mount heights 415 for the sensorwhen the sensor is angled at 0 degrees into traffic. In a preferredembodiment, the forward look down angle ‘A’ is in the range of 0 to 10degrees for creating high reflection of the emitted laser light. Areflective stripe may optionally be painted on the road surface atlengths on the road positioned at a lookdown angle of greater than 10degrees to increase the reflection of the emitted laser light. Areflective stripe is optionally employed if the pavement is very blackand therefore of low reflectivity. This ensures that there is asufficient amount of energy reflected back in the rain, where water isstanding on the pavement and tends to reflect energy away from thescanner (a mirror-like effect).

Referring back to FIGS. 1 and 2, in accordance with an embodiment, thesensor 105, 205 employs a pulsed time-of-flight rangefinder comprising adiode-laser transmitter and a silicon avalanche photodiode (APD)receiver in a side-by-side, off-axis configuration. For reference, anavalanche photodiode, as used herein, is a photosensor that generates alarge amount of current when struck by a small amount of flight due toelectron avalanche. The transmitter comprises the diode laser, itsdriver circuit, and a collimating lens. The optical receiver comprisesan objective lens, narrow-band optical filter, detector/amplifier, andthreshold detector, each coupled to each other.

The diode-laser, in one embodiment, is an InGaAs injection laser drivenby a diode driver to produce a pulsed output. A trigger pulse from ascanner control circuit triggers the laser at the requisite scan angles.In one embodiment, an ideal laser emission wavelength for the siliconAPD receiver is 904 nm. FIG. 5B shows a plurality of laser outputparameters in accordance with one embodiment of the present invention,including, but not limited to wavelength, maximum pulse width, maximumenergy per pulse, and average laser power. In one embodiment, the laserwavelength is 904 nm. In one embodiment, the maximum pulse width is 8ns. In one embodiment, the maximum energy per pulse is 64 nJ. In oneembodiment, the average laser power is 8 μW. The values provided aboveare exemplary values that reflect one embodiment of the presentinvention. It should be noted that these values may change and thatthere may be slight variations from unit to unit due to fluctuations inmanufacturing.

In accordance with one embodiment of the present invention, the sensoralso employs a rotating four facet cube to line scan, and thus,effectively directs the diode-laser pulse across its field of view(road) in a straight line. Thus, the four facet cube is employed as alaser collimator. Referring to FIG. 6, cube scanner 601 rotatescontinuously in one direction at a constant speed. The cube scanner 601comprises four sides, or facets, 601 a, 601 b, 601 c, and 601 d. In oneembodiment, the cube scanner 601 comprises one square or rectangularblock 691 formed by four facets 601 a, 601 b, 601 c, and 601 d and asecond square or rectangular block 692 also formed by four facets. Thesecond block 692, which is configured to receive and reflect transmittedenergy from the laser diode 602, and the first block 691, which isconfigured to receive and reflect the reflected energy from the road orvehicles, may be separated by a gap and may be physically coupledthrough an axis such that the two blocks 691, 692 are capable ofrotating relative to each other. The two blocks 691, 692 are mounted ona base 693 that may be separated from the second block 692 by a gap andcoupled to both blocks in a manner that permits them to rotate.

Each facet in each block 691, 692 comprises a reflective surface. Theangle between each facet and between each facet and the base of therespective block is 90 degrees. The four facet scanning cube enablesfour scans for each revolution. Conventional scanning systems use asingle mirror surface mounted at a 45 degree angle to the axis of thelaser, thereby allowing for only one scan per revolution of the mirror.By having four facets that are perpendicular to the laser axis, the cube601 of the present system provides for four scans per revolution. Assuch, the motor in the present system need rotate only at ¼^(th) thespeed of motors in conventional systems to achieve the same number ofscans. In addition, since the laser is pulsed at 1 degree of rotation inthe present system, the use of the four sided cube allows the laser inthe present system to be pulsed at ¼^(th) of the repetition rate ofconventional systems. This allows the present system to spin the motorfaster, scan more quickly, and pulse the laser at a lower frequency,which keeps the laser from over working or heating up to a harmfultemperature.

The rotating cube enables a fixed angular separation needed to scan thediode-laser 602 in a near straight line across an entire highway, eventhose having three or more lanes. A motor control mechanism 603 iscoupled to the cube 601 to facilitate rotation. Signals for motor speedcontrol 631 and facet position 632 are generated through a digitalsignal processor (DSP) 610 which, in one embodiment, is connected to acomputer through a suitable interface 660.

The DSP 610 also generates the laser trigger signal 633, which triggersthe laser driver 604 to activate the laser diode 602. Laser beam emittedfrom the diode laser is collimated using the lens 605. The beam 640 isdirected by the rotating cube scanner 601 and passed through atemperature controlled window 606 to scan the target vehicle. Becausethe window is capable of being heated and/or cooled, as required, thewindow is less susceptible to becoming foggy or blurry due tocondensation.

In one embodiment, optical detection circuitry converts opticalradiation reflected from the vehicle and/or road to an equivalentelectrical analog of the input radiation and subsequently, a logic-levelsignal. Thus, the laser beam reflected from the target vehicle 650 isagain directed by the scanning cube 601 through optical detectioncircuitry comprising a receiver objective lens 607, a filter 608 and anAPD detector 609, and finally to optical receiver 611. The logic-levelsignals are processed within a range counter logic 620 to yield digitalrange data 634.

In one embodiment, the pulsed time-of-flight measurements are read bythe digital signal processor (DSP) 610 and converted into distance/rangemeasurements. In an embodiment, a time-to-digital converter (TDC) isused as an integrated circuit on a single chip for time-of-flightmeasurements. This device enables the embedded software of the system todetermine the range of an object beneath the scanner by providing thetime period between a start pulse 635, which is generated when the laseris fired, and a stop pulse 636, which is generated when the reflectedenergy of the laser hits a target and reflects back to the scanner. Theuse of TDC provides better resolution, smaller size, simpler circuitry,lower power consumption, and lower cost when compared with prior arttime-to-analog conversion (TAC) and analog-to-digital conversion (ADC)multi-chip circuitry. This is because while TDC technology converts timesegments into a digital representation of that time, TAC technologyconverts a time segment into an analog value that must then be convertedinto a digital value. The TAC requires a relatively large amount ofelectronic circuitry to perform the task, whereas a TDC consists of asmall integrated circuit. TDC consumes approximately 0.005% of circuitboard real-estate compared to that needed by the equivalent TACcircuitry. In one embodiment, the range resolution is improved to be ±1cm, as against ±7.62 cm in prior art.

Further, the TDC can receive up to four return pulses from a singlelaser pulse. In accordance with an embodiment, by using two TDC chips onthe sensor, and switching back and forth between them, eight returnpulses from a single laser pulse can be received. In one embodiment,with the maximum range set at 35 ft, eight return pulses are received inno more than 70 nS or 0.00000007 seconds. It may be noted that this mayalso be achieved using a TAC, but the amount of circuitry required forthe purpose would take up at least 200 times more space on a circuitboard. This configuration improves the capability of seeing throughadverse weather conditions of rain, snow and fog by ignoring the returnsthat come in from the adverse weather conditions and using the returnsfrom the vehicles traveling under the sensor.

FIG. 5C shows a plurality of environmental factors and associatedperformance parameters in which the sensor is capable of performing inaccordance with one embodiment of the present invention. In oneembodiment, the environmental factors include, but are not limited totemperature, thermal shock, humidity, rain, snow loading, ice loading,wind loading, dust, vibration, shock, reliability, and maintainability.

In an embodiment, the provided laser geometry and collimating optics(four facet cube) provide a laser footprint with a characteristicdivergence of 82.6 u radians in the vertical axis and 16.5 m radians inthe horizontal axis. When the sensor 105 shown in FIG. 1 is mounted at7.65 m (25 feet) above the roadway, the width of roadway on the groundilluminated by the scanning 90 laser pulses covers 15.3 meter (50 feet).In one embodiment, when the sensor 105 is mounted at approximately 6 m(20 feet) above the roadway, a single laser pulse illuminates a 0.762 mm(0.03″) by 139 mm (5.49″) strip/footprint on the roadway therebyproviding high in-lane resolution and optimum cross-lane coverage fortwo to three lanes when the laser is pulsed once every degree of thescan angle. In another embodiment, if the sensor is mounted at a heightof 7 meters, for example, each range measurement for the laser beamilluminates a 0.508 mm (0.02″) by 115.6 mm (4.55″) strip/footprint onthe pavement.

Referring to FIG. 3, in another embodiment when the sensor is mounted ata height of 25 feet, the laser footprint falling on the pavement isapproximately 0.15 in (3.6 mm) in a vertical direction 310 and 4.6 in(117 mm) in a horizontal direction 305. It should be appreciated thatthe footprints appear as stripes along the pavement. These “striped”pattern footprints are formed during scanning and appear as successivestripes across the road that just touch end to end therefore providing acontinuous line of detection. The striped footprint pattern is a resultof the use of a diode laser where the output facet of the light emittingchip is rectangular in shape.

In one embodiment, the system generates 90 pixels for each scan, whichline up across the scan line with very little gap in between. With themounting geometry of FIG. 1, the pixel-to-gap-ratio is more than 18.8.The gap between subsequent pixels is approximately 6.6 mm (0.26″) atthis mounting height. Hence, any shape larger than the size of the gap,e.g. a 5 cm (2″) tow bar, will be detected by at least one pixel of thesensor, thereby enabling vehicle detection accuracy exceeding 99% in oneembodiment . This type of laser footprint that appears as a continuousscan line allows, for example, for detection of a trailer and also itsattachment to the towing vehicle. It may be noted that the highpixel-to-gap ratio is achieved by use of the striped, continuous scanline design of the present system. In one embodiment, the laser ispulsed at 1 degree intervals. With the interval known, the appropriatelaser width and optics are selected to yield a specific beam divergence,such that the width of the laser increases in size at the exact rate asthe angle separation. One of ordinary skill in the art would appreciatethat conventional laser scanners have a round footprint, as opposed to astriped, continuous scan line of the present invention, and henceconventional scanners produce a great amount of overlap as the rangeincreases, whereas the presently disclosed embodiments minimize overlap.

FIG. 5A is a table showing a plurality of performance parameters for thesensor of the present invention in accordance with one embodiment, suchas scan rate, range accuracy, angular resolution, etc. In accordancewith one aspect of the disclosed inventions, the minimum range gate ofthe sensor can be set and/or customized by a customer. This enables thecustomer to set the sensor to ignore any returns up to a predeterminedrange and only process distances beyond that range. In one embodiment,the minimum range gate can be set from 0 ft up to 25 ft in ⅛ ftincrements. This also can be used to prohibit adverse weather conditionsfrom causing short ranges (false alarms) that could distort theresultant three-dimensional profile of the vehicle being scanned.

In one embodiment, the customer can customize the number of pulses thatwill occur within each scan. In one embodiment, the customer cancustomize the angle of the scan. In one embodiment, the angle of thescan can be adjusted from a maximum of 90 degrees to a minimum of 20degrees. A person of ordinary skill in the art would appreciate thatseveral other parameters may be adjusted by software according to userpreference.

Persons of ordinary skill in the art should appreciate that since sensorof the present invention measures ranges in a single plane, the speed ofthe moving vehicle is optionally captured by other sensor(s) in order toallow a calibrated three-dimensional measurement. However, even anon-calibrated 3-D profile allows valuable information about the vehicleprofile and enables the computer to easily distinguish between, forexample, a truck and a bus. One of ordinary skill in the art wouldappreciate that the various types of vehicle classifications, such astruck, bus, pickup, car, van, sedan, convertible, compact, etc. is onlylimited by the complexity of software, and hence the system can beadapted to classify vehicles into any number of categories. In oneembodiment, the system is able to classify up to 12 classes of vehicles.In one embodiment, the sensor of the present invention automaticallyinitializes the ranging process upon power-up, and its self-calibrationprocess eliminates the need for any field adjustments on initialization.

In accordance with another aspect, the system of the present inventionhas the capability of reporting the intensity of reflected objects alongwith the range data. The purpose of capturing the reflected intensityfor every pixel across the scan line is to perform range correction andto provide additional data to the classification algorithm indetermining the class of vehicle. In addition, the intensity data can beused to improve the classification and detection of vehicles duringadverse weather conditions. In the event of reflection from poolingwater or oil on the ground or reflection from a vehicle wind screen, therange reported by the sensor can be significantly longer than the actualrange to the reference surface. Therefore the capture of intensity datahelps the user understand why the data reported from the sensor appearsto be wrong. Another example of troubleshooting is the case where theoptical alignment of the sensor has been altered for unknown reasons. Inthis case, the intensity reflected back to the scanner may be too lowfor proper and consistent ranging. Accordingly, analyzing the range andintensity data enables system operators to identify the cause of reducedsensor performance.

The above examples are merely illustrative of the many applications ofthe system of present invention. Although only a few embodiments of thepresent invention have been described herein, it should be understoodthat the present invention might be embodied in many other specificforms without departing from the spirit or scope of the invention.Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention may be modifiedwithin the scope of the appended claims.

We claim:
 1. A system for determining the three-dimensional shape of avehicle, the system comprising: a distance sensor comprising a lasertransmitter and a photodetector, for generating a plurality of laserbeams and for detecting a plurality of reflected beams, each of saidreflected beams corresponding to one of the plurality of generated laserbeams; a scanning mechanism, positioned relative to the distance sensor,for collimating each of said generated laser beams across the vehicle,wherein said scanning mechanism comprises a four facet cube, each havinga reflective surface, wherein said four facet cube is positionedrelative to the distance sensor such that it is adapted to reflect thegenerated laser beams, and wherein said scanning mechanism furthercomprises a scanner control circuit in data communication with saiddistance sensor to trigger the generation of the laser beams to createpredefined scan angles; and a processing system to determine distanceranges from the sensor to portions of the vehicle using time-of-flightmeasurements derived from timings of said generated laser beams andreflected beams, when the vehicle travels within a sensing zone of thesensor and to determine a three-dimensional shape of the vehicle basedon distance ranges.
 2. The system of claim 1 further comprising atime-to-digital converter (TDC) for time-of-flight measurements.
 3. Thesystem of claim 2 wherein the TDC is adapted to receive up to fourreturn pulses from a single laser pulse.
 4. The system of claim 2comprising at least two TDCs.
 5. The system of claim 1, wherein the fourfacet cube rotates continuously in one direction at a constant speed. 6.The system of claim 1, wherein the four facet cube enables four scansfor each revolution.
 7. The system of claim 1, wherein the systemgenerates a plurality of laser footprints and wherein said laserfootprints appear as stripes that touch end to end and provide acontinuous line of detection.
 8. The system of claim 1, wherein thedistance range resolution of the system is ±1 cm.
 9. The system of claim1, wherein the limits of distance range measurements are customizable.10. A method for determining a three-dimensional shape of a vehiclepassing through a sensing zone of a ranging sensor comprising a lasertransmitter and a photodetector, the method comprising: scanning aplurality of laser beams across the vehicle using a scanning mechanismcomprising a four facet cube, said four facet cube having reflectivesurfaces that are used to direct the laser beams across its field ofview in a straight line, said scanning mechanism further comprising ascanner control circuit that triggers the laser at predefined scanangles; determining a distance range from the sensor to portions of thevehicle using time-of-flight measurements; and processing the distancerange data for each scan angle to determine the three-dimensional shapeof the vehicle.
 11. The method of claim 10, wherein the four facet cuberotates continuously in one direction at a constant speed during scan.12. The method of claim 10, wherein the scanning cube is adapted toproduce four scans for each revolution.
 13. The method of claim 10further comprising using a time-to-digital converter (TDC) fortime-of-flight measurements.
 14. The method of claim 13 wherein the TDCis adapted to receive up to four return pulses from a single laserpulse.
 15. The method of claim 13, wherein the system comprises at leasttwo TDCs.
 16. The method of claim 10, wherein the system generates aplurality of laser footprints during a scan and wherein each of saidlaser footprints appear as stripes that touch end to end and provide acontinuous line of detection.
 17. The method of claim 10, wherein thedistance range resolution of measurements is ±1 cm.
 18. The method ofclaim 10, wherein the limits of distance range measurements arecustomizable.
 19. The method of claim 10, wherein when the scannercontrol circuit triggers a laser pulse once for every degree of scanangle.